(1) Field of the Invention
The present invention relates to telephone subscriber line circuits used in digital telephone offices and more particularly to the use of a Hall effect device in a line circuit to generate supervisory and transmission signals.
(2) Description of the Prior Art
Typically line circuits use a line relay to perform the supervisory functions of loop, dial pulse and ring trip detection, and further require the use of a cutoff relay to disconnect the line relay when the line circuit operates to transmit subscriber signals. More recent line circuits have used Hall effect devices to perform the supervisory functions. However, they still require either a cutoff relay, or balanced coils and a capacitive shunt between tip and ring leads, to generate the magnetic flux for the Hall effect device, and to provide longitudinal balance. Examples of such line circuits include R. J. Sabon's U.S. Pat. Nos. 4,166,928 and 4,191,859 issued on Sept. 4, 1979 and Mar. 4, 1980 respectively. However, such circuits require longitudinally balanced coils to generate the magnetic flux for the Hall effect device, and also these circuits could only use the Hall effect device for the supervisory function of the line circuit. Accordingly, it is the object of the present invention to provide a circuit which can perform both the supervisory and transmission functions of a line circuit under control of a Hall effect device and without the need for longitudinally balanced coils or a cutoff relay.